pinctrl: lantiq: Remove unsued declaration ltq_pinctrl_unregister()

Commit 3f8c50c9 ("OF: pinctrl: MIPS: lantiq: implement lantiq/xway pinctrl support")
declared but never implemented it, so can be removed.
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230825092706.14680-1-yuehaibing@huawei.comSigned-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/pinctrl-lantiq.h

@@ -198,5 +198,4 @@ enum ltq_pin {
 
 extern int ltq_pinctrl_register(struct platform_device *pdev,
 				   struct ltq_pinmux_info *info);
-extern int ltq_pinctrl_unregister(struct platform_device *pdev);
 #endif	/* __PINCTRL_LANTIQ_H */